Solvent extraction of U(VI) from media containing competing anions and metal cations by a calix[6]arene bearing carboxy groups

The selective extraction of uranium by a macrocyclic molecule, 5,11,17,23,29,35-hexa-tert-butyl-37,39,41-tri(carboxymethyl)-38,40 , 42-trimethoxy-calix[6]-arene has been studied in a two-phase solvent-extraction system. The first part of this study concerns the determination of extraction mechanism. The variation of the extraction parameters, performed for two different ionic strengths (I=0.1 and I=3), allows us to propose an extraction scheme. The second part of this work concerns the selectivity of calix[6]arene towards uranium. Some competing metal cations present in urine have been taken into account. In the aqueous phase (spiked with exp 230 U), divalent metals such as Zn exp 2+ , Cu exp 2+ , Mg exp 2+ , Ca exp 2+ were added. The results show that the uranium extraction is independent of the cation concentration, whatever the pH and the ionic strength are. The competition between the chosen calix[6]arene and other complexing anions, such as phosphate or sulfate, has been investigated. The results indicate that the uranium-calixarene extraction constant is higher than the uranium-anion complexation constant.